For Begonia plants, temperatures that drop below 50°F (10°C) are considered the primary threshold for cold stress. At this point, the plant's metabolic processes begin to slow down significantly. Enzymatic activity, which is crucial for energy production and nutrient uptake, becomes less efficient. More critically, cell membrane integrity is compromised. These membranes, composed of lipids, start to lose their fluidity and can become more rigid, leading to potential rupture. This cellular damage is the fundamental reason why cold is so dangerous; it disrupts the very building blocks of the plant's structure from the inside out, making recovery difficult if the exposure is prolonged.
The most critical and damaging temperature range for Begonias is at or below 32°F (0°C). This is the freezing point of water, and it presents an acute, physical threat. The water content within the plant's cells freezes, forming sharp ice crystals. These crystals literally pierce and shred the cell walls and organelles, causing irreversible damage. This damage manifests visibly as blackened, mushy, and translucent leaves and stems—a condition often referred to as frost damage. For a Begonia, a single night of exposure to freezing temperatures is often fatal, as the structural integrity of its tissues is destroyed beyond repair.
Even if temperatures never reach freezing, sustained exposure to the "chilling" range between 32°F to 50°F (0°C to 10°C) can cause significant harm known as chilling injury. The plant may not die immediately, but it will exhibit clear signs of distress. Growth will be severely stunted as metabolic functions grind to a near halt. Leaves may begin to drop prematurely, and the vibrant colors often fade. Perhaps the most telling sign is the appearance of necrotic (dead), brown spots or lesions on the leaves, particularly along the margins and between the veins. This is a direct result of the cellular membrane damage occurring at a microscopic level, leading to cell death in localized areas.
It is crucial to note that cold tolerance varies significantly among the vast Begonia genus. Generally, hardy begonia species (Begonia grandis) can survive much colder winters, dying back to the ground and re-emerging from tubers in spring. However, the common bedding and houseplant varieties, such as wax begonias (Begonia semperflorens) and tuberous begonias (Begonia × tuberhybrida), have virtually no cold tolerance. Rex begonias (Begonia rex-cultorum) and other rhizomatous types, prized for their ornate foliage, are particularly sensitive to chilly conditions and drafts. For these tender varieties, maintaining a consistent temperature well above 60°F (15°C) is ideal for healthy growth.
From the plant's perspective, a sudden drop in temperature is far more damaging than a gradual one. A slow acclimation process allows the plant to make physiological adjustments, such as increasing sugar concentration in its cells to act as a natural antifreeze. For gardeners, this means paying close attention to fall weather forecasts and moving container Begonias indoors well before the first frost. When indoors, protecting plants from cold drafts from windows or doors is equally important, as these can create localized microclimates that are much colder than the overall room temperature, subjecting the plant to the same chilling injury it would experience outside.
